Definitely agree, when you do get access to the bank account you should scrutinise the details, not just for the period since your dad's death, but also for the period he was in hospital and the time he was ill and not leaving the house. It is possible that they were abusing him financally while he was vulnerable. Perhaps they did not, but their behaviour since his death, and their reaction to you taking over the adminstration of his affairs, does ring a lot of alarm bells.
Hope for the best and plan for the worst. That means DEFINITELY don't give them any money, as at this point, you don't know what they may have already taken (from the house, from the bank, any other bank accounts, any other assets).
As PP have said, if he wanted aunty and other to be his beneficiaries, he would have made a will.
Sending you hugs and strength while you go through this. My father (that I didn't see often) also died intestate. I was his next of kin (and very young) but not the only beneficiary. The people who were smarming up to me and being friendly had in fact robbed him in between him dying and me getting keys to his flat to sort things out. It took me quite a while to piece things together and being young, I didn't know how to deal with things (so I did nothing). But it hurt to know people he trusted behaved in that way. There were also disputes over how the estate shoudl be split, and it was very unpleasant. As you are the only child it should be a simple affair but I am sorry that you have to deal with your family's bad behaviour while you have just lost your father, with the added upset of not having known that he was so unwell. x